1. Filed of the Invention
The present invention relates to a novel nematic liquid crystal mixture and a liquid crystal device comprising the same. In particular, the present invention relates to a liquid crystal mixture which has an effectively widened temperature range for the nematic phase and a low viscosity, and a liquid crystal device comprising the same, for example, an optical shutter, a display device such as a super twisted nematic (STN) liquid crystal device or a polymer dispersed liquid crystal (PDLC) device and the like.
2. Description of Related Art
The liquid crystal displays (LCD) have been widely spread as displays for personal computers, word processors and the like, because they have various characteristics such as low consumption of electric power, thinness, light weight, driving at a low voltage and the like. Among the liquid crystal devices one of typical examples of which is a liquid crystal display, a matrix type liquid crystal display which stores a large amount of information has two driving systems, that is, an active matrix system and a passive matrix system.
The STN liquid crystal devices are driven by a multiplex system, and in these years widely used as color LCD for notebook type computers since their response speed and viewing angle have been improved greatly.
TFT liquid crystal devices are installed in high performance personal computers since their image quality and yield have been improved, although they are more expensive than the STN liquid crystal devices.
However, the STN liquid crystal devices have advantages over the TFT liquid crystal devices in the cost, while they still have the insufficient viewing angle and response speed. The STN liquid crystal devices have some problems to be solved for further increase of the picture area, price reduction and increase of a density. In particular, the increase of response speed is essential for further increase of the picture area and density and for displaying video motion pictures.
A liquid crystal mixture contains a number of components for the optimization of a plurality of properties, and each component has its specific properties. For example, high speed STN liquid crystal devices having a reduced cell gap have been developed. A liquid crystal mixture used for such the high speed STN liquid crystal devices should contain a component having a large birefringence .DELTA.n in addition to the conventional components such as one having the nematic phase in a high temperature range, one which can impart the dielectric anisotropy to the liquid crystal mixture.
In general, a mixture containing a component having the large .DELTA.n has a high viscosity .eta., and is less suitable for increasing the response speed. That is, it is known that a ratio .eta./.DELTA.n.sup.2 (a responsiveness parameter) is proportional to the response time, and therefore the liquid crystal having the smaller ratio .eta./.DELTA.n.sup.2 has the better response properties (see Tatsuo Uchida, "Next Generation Liquid Crystal Display Techniques", KOGYOCHOSAKAI, page 136). Thus, liquid crystal mixtures having the larger .DELTA.n and the smaller ratio .eta./.DELTA.n.sup.2 have been sought. It is essential for the nematic phase to have a wide working temperature range including room temperature, and therefore a component which can effectively widen the lower and upper temperature limits is important.
Furthermore, it is essential to lower a threshold voltage from the view point of the low consumption of electric power.